Instantaneous frequency monitor



y 26, 5 w. R. WILSON ETAL 2,640,106

INSTANTANEOUS FREQUENCY MONITOR Filed May 15, 1950 I3 Keyer I Mo Fig I I2 29 Frsegli fggy Electronic Discriminator Signal Source Switch Detector l8 2| m Amplifier 17-0 -|5 I6 22 620 Calibrated Calibrated Oscillator Oscillator Mark Frequency Space Frequency Cathode Ray Tube Fig. 3. 64; elite)? g f f r r n t 1; I; s 0 s 0 iscrimlna or f0 Calibrated Oscillator Frequency fs Space Frequency or Interval frn =Mark Frequency or Interval William R. Wilson 8t Junius B. Reynolds.

Patented May 26, 1953 INSTANTANEOUS FREQUENCY MONITOR William R. Wilson, Ellicott City, Md., and J unins- B. Reynolds, Princeton, N. J assignors to Westinghouse Electric Corporation, East Pittsburgh, 1 Pa., a' corporation of Pennsylvania Application May 15, 1950, Serial No. 162,098

4 Claims. (Cl. 17 8-69) This invention relates to methods of, and apparatus for, measuring the frequencies of frequency modulated signals, and relates more particularly to the monitoring of frequency-shift keying systems.

In frequency-shift keying systems, a carrier wave is frequency modulated by shifting the carrier frequency a predetermined amount on either side of a normal or mean frequency. When ordinary hand keying is used, the frequency emitted during key-up intervals is known as the space frequency and may be represented instantaneously as equal to the carrier frequency minus the deviation frequency, the amount by which the instantaneous frequency deviates from the carrier frequency. During key-down intervals, the frequency emitted is known as the mark frequency and may be represented instantaneously as equal to the carrier frequency plus the deviation frequency.

At normal keying rates, the emitted frequencies are different from those emitted during hand keying, and the emitted energy is concentrated in sidebands spaced on both sides of the carrier frequency in multiples of the fundamental modulation frequency. The sidebands nearest the space and mark frequencies contain most of the emitted energy, and since at higher keying rates, these sidebands may not fall on the space or mark frequencies, it is difficult to measure the space, mark, or deviation frequencies with a high degree of accuracy.

One method of measuring the space, mark, and deviation frequencies at normal keying rates, which has been proposed, involves the measurement of the output wave form from a discriminator type, frequency modulation detector. When the output of the discriminator is calibrated in terms of output frequency, it is possible to make approximate measurements by observing the output wave form on the screen of a cathode-ray oscilloscope. However, it is not practical to measure this output to the accuracy which is frequently required, and even if such accuracy were possible, with the present state of the art it is difficult to build a circuit with the required stability.

This invention provides for the accurate measurement of the instantaneous frequency of a frequency modulated signal. of theinvention for the measurement of space and mark frequencies during frequency-shift keying, the frequency to be measured, and the frequency from a calibrated oscillator operating inthe range of the frequency to be measured, is

In one embodiment 2. fed to an electronic switch which alternately applies each of the two frequencies to discriminator type detector. The electronic switch passes the signals for a period of time which may be slightly shorter than either the mark or space period. The output of the discriminator is amplified and applied to the vertical plates of a cathode-ray oscilloscope in which the sweep is synchronized with the keying. The resulting image on the oscilloscope screen consists of two separate patterns alternately superimposed one over the other. The amplitude of one will represent the frequency to be measured, while the amplitude of the other will represent the frequency from the calibrated oscillator. The amplitudes of the two patterns are made equal by adjusting the. frequency of the calibrated oscillator. At that time, the frequency of the oscillator will be the same as the frequency of the signal being measured, and can be read directly from the calibrated scale of the oscillator.

An object of the invention is to accurately measure the instantaneous frequency of a frequency modulated signal.

Another object of the invention is to accurately measure the instantaneous values of the mark and space frequencies of a frequencyshift keying system. Another object of the invention is to provide an accurate, stable monitor for the measurement of the instantaneous frequencies of a frequency modulated signal.

The invention will now be described with reference to the drawing, in which:

Fig. 1 is a schematic block diagram of a frequency monitor embodying the invention, as used for measuring the space and mark frequencies of a frequency-shift keying system;

Fig. 2 illustrates the manner in which the patterns, representing the frequency to be measured and the calibrated frequency, appear on the screen of the oscilloscope of Fig. 1, and

Fig. 3 illustrates the circuit of an electronic switch which may be used on our monitor.

The keyer I 0 frequency modulates by means of a rectangular wave form, the carrier wave produced by the signal source H of a conventional frequency-shift keying system. The output of the source is connected to the input of the electronic switch l2. The keyer I0 is also connected through the connection H! to the switch I2 for supplying a synchronizing signal thereto, and through the connections l3 and 25 to the cathode ray tube 24. The input of the switch 12 is also adapted to be connected by a The output of; the switch. :2 co the input of: the conventional discriminator oeoscillator designed to generate: asignal with the:

required accuracy and having a range of frequencies in'the range of the space frequency of the system, and has a knob, 21021501: adjusting the frequency of the signal it generates, and has the scale 2| and the cooperating indicator 22 for indicating the frequency which the oscillator is adjustedtol generate.

ectedi to teeter 25., the output of which. is connected. to the input of the high; gain; amplifier" 23- The output. or the; amplifier 2.3: is connected to the vertical plates of the conventional; cathodearay oscilloscope 2a, the sweep: of the: oscilloscope be ing synchizornzed, through. the connections I 3 z'ifi to thekeyer F- The: electronic switch I23. illustrated schematil cally in Fig. 33, comprises two pentagri'd tubes 33 ana tic, a ti'i'od etube 33" anda double poledouble throw switch 33 The tube: 30 has a plate 3?, a cathode 38*; and grids 33, 40:, M, 42 and 43. biasing circuit con-ionising a resistance 425 and a. condenser 46 is: connected be tween the cathode 33 and ground. The grids it and are connected together and thegrid i3 is connected tattle-cathode 33. The output from the calibrated oscillator (Fig. 1 isconnectect to the grid N The tube. 3F has a plate FF, a cathode 43, and grids 49, 5t, 5|, 52 and 53 A biasin circuit comprising aresistance 55'- and a condenser- 56 is connected between the (596171 1068? 4 8 and ground; The: grids 553 and 52- are connected tcgctherand the grid 53 is connected to the cathode 4 8. The output from the.- frequency shifted signal source H (Fig. I')'- is connectedtogrid 5|.

The: plates 3-? and 4'? are supplied with. suitable 13+ potential across the impedance comprising resistor 30, capacity 6 1 and inductance 62 carried in paraliel. The screen grids 4'0 and 53- are supplied with suitable B r potential across resistor 64 The condenser 35- is a by-pass for hie-screen grids to 30-. The output from the plates 31 and GT is connected to the input of thediscriminator detector (Wig. 1').

The triode tube 33 functions as an inverter and it comprises a plate 38; a cathode G3, and a grid Ill. The plate 38 is-connected to a. suit-- able source of B+- potential through a resistor 72. The-cathode is connected to ground through an un'bypasscd resistor The double pole-double-th1=ow switch 35 com prises. three pairs 01 contacts or terminals l-G', ll-,1 T8 and 13",. Miami a. bridging member 81 The. grid. 39 of tube 311? is connected to terminaala 1150f switch. 315: through. conductor 85; The gritl 4a of the" tube. 3b is connected to terminal i i?- of the switch. 37:: through conductor 83.

. Theoutput-from the keyer to: (Fig; L) iacom nected: to: the grid in? on tube 33 through. conto terminal 16 of switch 35 through coupling capacitor 90. The cathode .59 of tube 33 is connected to terminal 15 of switch 35 through coupling capacitor 9|.

The rectangular keying signal from the keyer I0 is passed through the inverter 33 to obtain equal butout" or phase. keying. signals; These signals are in turn applied to the grids 39 and 49 of the tubes 30 and 3|, respectively, alternately permitting the tubes 30 and 3| to pass the. signals imposed upon the grids 4| and 5|. This produces in the plate circuits of the tubes 38 amli 3A5 alternate calibrated oscillator and frequency-shined signals in synchronism with the keying signals;

With the bridging member SI, of switch 35, bridging terminals 19 and 80, the tube M will pass only the space interval signals from the frequency shifted source ll (Fig. 1), With the bridging member 8|, oii switch 3|, bridging terminals F3 and. 7.3,. the tube. 3i will case only the rear the. oscilloscope: The lower Iii] suppliesthe sweep frequencies The electronic switch [1' passes. each. signal for a period of time which may be; align y shorter than: the mark period;

The resui'tii ig image shown on! the screen- 23 ot the oscilloscope willconsist of two separate pat terns 2t: and to alternately superimposed one upon. the other, as: illustrated by Fig; 2-, inwhich the amplitude a. the pattern 2-8 represents the marl: frequency and the amplitudebof" the pattern Firepwse ts the frequency" of the signal from: the oscillator l t. The control knob lzliot the osciliator 15 is thenad j usteduntil the amplitude: Iii equals the amplitude c at which the marl: frequencywill be equalto' the frequency of the oscillator l-ii, which frequency can be readfirom: the-position of the indicator [9 along the calibrated scale I 8;

By moving the switch H over to disconnect the osciil'atcu 8 5 l 'rom the electronic switch I 2, and: to: connect the oscillator l't to the electronic switch, the: space frequency can: bedetermined following the procedure described intheforegoing. Thepatterns-ontheoscilloscope screen with. howeven. he inverted;

' Cmly that sector of theoutput-voltage range or the: discriminator detector 2? corresponding; toithe desired frequency need be'amplifiedin the amplifier an. at the two outputs.- firomi the detector 29 are magnified: in the. amplifier: 23;. and. this; results in the: greatly increased; accuracy oif'the men-- itor.

Another important advantage provided by this invention. is: that dueta the fact that the di criminato'r detector output iscontinuously calibrated, chan'gesrinx its. operating conditions such as dertuning. will; have: effect on either accuracy/ onstability.

; Win-1e the: invention has been described in. cons doctor Maine-plate 68. tube '3i3- is-connectedi 7 i cl ioncoththe moni mlinaor: trequency shiited The di'frerence'il-r the ampli inid'e keying systems, it is applicable to the monitoring of other types of frequency modulation systems.

We claim as our invention:

1. A monitor for measuring the frequency of a frequency modulated signal source comprising: an oscillator; a detector; switch means for alternately applying the signal from the source and the signal from the oscillator to the input of said detector; means for providing one visual indication of the voltage corresponding to the signal from the source, and for providing a second visual indication of the voltage from the detector corresponding to the signal from the oscillator; control means for adjusting the frequency of the oscillator until the amplitude of the visual indications is the same, and indicator means for determining the frequency of the oscillator.

2. A monitor as claimed in claim 1 in which the indicator means for determining the frequency of the oscillator is a calibrated scale for indicating directly the frequency of its output.

3. A monitor for measuring the frequency of a frequency shift keying system having a keyer comprising: an oscillator; a detector; means for alternately switching the signal from the source and the signal from the oscillator to said detector; a cathode-ray oscilloscope connected to the detector and to the keyer so as to have a sweep synchronizing with the keyer, and. to provide patterns on its screen which have amplitudes corresponding to the voltages from the detector resulting from the two signals supplied thereto; means for adjusting the oscillator until the amplitude of the pattern on said screen corresponding to the frequency of its signal is equal to the amplitude of the pattern on said screen corresponding to the frequency of the signal from said source, and means for determining the frequency of the oscillator.

4. A monitor as claimed in claim 3 in which the means for determining the frequency of the oscillator is a calibrated scale for indicating directly the frequency of its output.

WILLIAM R. WILSON. J UN'IUS B. REYNOLDS.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,897,204 Loewe Feb. 14, 1933 2,178,074 Jakel Oct. 31, 1934 FOREIGN PATENTS Number Country Date 891,937 France Dec. 17, 1934 OTHER REFERENCES Notes on Audio and Supersonic Frequency Measurements, by A. K. McLaren in Radio, August 1945, pp. 46, 49. 

